Why bother?

When I first dove into D-STAR, I knew nearly nothing about digital voice and quickly found myself drowning in a big bowl of bewilderingly murky information soup. SOS!

So if it's that bewildering, why even bother?

I'll jump a bit ahead here and share one tidbit: at one point early in my exploration of D-STAR, I linked to a reflector and heard a guy in San Diego, California chatting with a chap in Yorkshire, England. That was the moment I became hooked!

Just think of it: worldwide communication with a Technician class license, a bit of effort and learning, and some fairly simple equipment.

Of course, the real goal isn't to just listen but to actually chat with people. This is the story of how I got there.

Today's amateur radio experimenter is as likely to use a keyboard as a soldering iron for experiments, and as a digital enthusiast, I can only cheer and encourage you to get involved and have some fun.

Dedicated to the faint of signal

This article is written from the point of view of someone who doesn't live within range of a digital voice (DV) repeater, someone for whom a Personal Access Point device (hotspot) is an important key to accessing a digital voice system, and is a gift that opens doors to the whole wide world.

While much of this article applies to everyone who wants to use digital voice, especially D-STAR, a significant portion is focused on the ham who wants to put together and use a digital voice hotspot, which in some respects is quite different from using regular D-STAR.

This article unfolds over time, loosely following the stages of my own journey of discovery into D-STAR; however, I'm also going back and revising it as I continue to learn more.

Caveat

It's important to acknowledge that because digital voice often relies on an internet connection, it may not be a reliable mode of communication during a major emergency when the internet might be overwhelmed or offline, as happened to us during the flood that hit our area in 2013. Beyond that, it seems like an awesome advancement that continues the long history of exciting amateur radio innovation.

Disclaimer

These are my personal notes and opinions based on my experience using D-STAR, as well as by learning from what others are sharing. I've tried to be accurate, but there may be things I'm mistaken about. Please let me know if you come across anything needing correction.

1) What is digital voice?

The simple answer is that digital voice (DV) uses digital rather than analog audio. But what does that really mean? For me, it was all quite murky when I first started looking into it, and it took me quite a bit of exploring and head scratching to begin figuring it out.

This section provides an introduction to the basics of digital voice and hotspots, including links to some helpful websites, articles, and other content, in case you want to dive in more deeply. You also can find all the links together on the Notes page at the end of this article: Links to helpful resources.

1a) Alphabet soup

As I began flailing around in the digital voice murkiness, one of the first things I figured out is that it's actually a bit like a big bowl of alphabet soup … it's absolutely crazy how many acronyms are floating around!

Even worse, sometimes the acronyms are spelled out differently, for example, D-STAR vs DStar, or DVMEGA vs DV MEGA vs Dv-Mega, which makes searching for information more challenging. In this article, I try to consistently use the official or standard version of acronyms, except when I quote others.

Other times, it isn't easy to determine what an acronym actually stands for, for example, in relation to digital voice I've seen DCS defined as Digital Call Server, Digital Communication Systems, and Digital Call Service, and I'm still unsure which is correct. (Perhaps it stands for Darn Confusing Stuff? ! )

Since there's no avoiding this craziness, you just have to accept it as part of the price of admission. And don't worry, by the end of this article, you'll be speaking "Alphabetsoupese" as fluently as is possible!

1b) Multiple systems

The next thing I understood is that D-STAR is just one of several competing digital voice systems being developed, though it's one of the earliest, and the first to be developed specifically for amateur radio use.

The Japan Amateur Radio League began development work on the Digital Smart Technologies for Amateur Radio (D-STAR) standard in the late 1990s and published it in 2001. It started to gain traction in the U.S. around 2006.

The D-STAR standard has been adopted primarily by two first-tier amateur radio equipment makers, first by Icom, the D-STAR trailblazer, and more recently by Kenwood. By now, it's being used by tens of thousands of amateur radio enthusiasts worldwide.

It's also an open standard (with the exception of the vocoder chip, which is discussed in the following note: Alphabetsoupese – example 1), so it's being used for lots of experimentation, and that means there's a bunch of interesting homebrew and small manufacturer hardware and software available.

Among the other digital voice systems being developed or adapted for use by hams are:

DMR, an open commercial standard developed and governed by the European Telecommunications Standards Institute, with equipment by Motorola, Hytera, Tytera, and others.

P25, a standard developed for North American public safety services. Governed by the Telecommunications Industry Association and others.

NXDN, an open commercial and public safety standard initially developed by Icom (implemented as IDAS) and Kenwood (implemented as NEXEDGE), and now overseen by members of the NXDN Forum.

There's also a totally open system being developed for amateur radio that is a combination of FreeDV software and an open source speech codec (vocoder), Codec 2. There's a version of FreeDV available for HF, but it hasn't yet come to the VHF and UHF frequencies (as of the end of 2017).

Alphabetsoupese – example 1

You May Not Necessarily Need To Know This (YMN³TKT), but you'll come across these acronyms.

One thing the main DV systems being used by hams have in common is that they all compress digital audio using the Advanced Multi-Band Excitation (AMBE) vocoder chips and software made by Digital Voice Systems, Inc. (DVSI).

Finally, the compressed digital audio is modulated onto the carrier wave. I've added a red arrow pointing at the modulated carrier wave, which I'll talk about a bit more in the next note.

There are a few different versions of the DVSI AMBE Vocoder in use: AMBE, AMBE+, and AMBE+2. The newer versions add improvements like better sound quality and error correction.

Because the DVSI AMBE Vocoder software is proprietary, this is one area of digital voice—and the only area of D-STAR—that hams can't hack. For obvious reasons, some people don't like that.

How does the vocoder work?

Simply digitizing an analog voice waveform actually results in needing more bandwidth than the analog original. To solve this conundrum, DVSI built upon research started at MIT to create the Multi-Band Excitation-based AMBE Vocoder, which both minimizes bandwidth requirements and maximizes human voice fidelity.

Instead of just digitizing a waveform, we can recognize that the human voice has some very predictable characteristics, and we can exploit those characteristics to dramatically reduce the digitized bandwidth while maintaining that "human voice" sound

Human speech has two major sound components, voiced and unvoiced. Voiced energy is periodic in nature, containing tones or frequencies, while unvoiced energy is like noise. To better understand this concept, say the word "wash" out loud. The first part of the sound is voiced at a relatively constant frequency, changing in its harmonic content, while the "sh" ending is unvoiced and essentially a burst of noise.

To simplify the complex and clever work being done: the AMBE vocoder splits the human voice signal into frequency bands, then analyzes the audio energy of the major voice sound components in each band, creating a bandwidth efficient summary of the characteristics.

The amazing thing to me is that it sounds so good; I find it totally easy to identify people when listening to their digital transmissions, even if they're using radios with the older AMBE and AMBE+ chips.

1c) Transceivers, repeaters, and reflectors (oh my)

Just as you can use analog transceivers in FM mode, you can use digital voice-capable transceivers in DV mode to talk directly from radio to radio simplex ¹ or via a repeater (as long as it's a digital voice repeater).

It gets even more interesting when digital voice repeaters are bridged together, enabling groups of two or more people, even far flung, to participate in something that's like a conference call.

In the D-STAR world, this technology is called "Reflectors" (transmissions are reflected to all repeaters linked to the reflector), while DMR calls it "Talkgroups" and System Fusion calls it "Rooms."

Linking to reflectors to participate in a call or net is a big part of what playing around in the worldwide D-STAR playground is all about.

Here, too, there are multiple systems being developed and used simultaneously. Within just the D-STAR world, there are four main types:

REF – The DPLUS reflector system, a closed proprietary system developed by Robin Cutshaw, AA4RC, is the first generation of D-STAR reflectors and still much in use, especially in English-speaking countries. An example is REF001 in London, referred to as D-STAR's "Mega Reflector." REF directory.

XRF – The Dextra X-Reflector system, originally created by Scott Lawson, KI4KLF, is the second generation of D-STAR reflectors and is open source. An example is XRF720, which links several Colorado statewide D-STAR repeaters. XRF directory.

DCS – The Digital Call Server reflector system, developed by Torsten Schultze, DG1HT, is an even newer closed system that is being used worldwide. An example is U.S. Reflector DCS006. DCS directory.

XLX – The XLX reflector system, being developed by Jean-Luc Boevange, LX3JL, and Luc Engelmann, LX1IQ, is an open system and the newest, which they describe as "the first and only multiprotocol Reflector system until now and supports all standard D-STAR protocols like DCS, Dextra and DPlus fully transparent." XLX directory.

Same difference

While there are some differences between the three main digital voice systems being used by hams—D-STAR, DMR, and System Fusion—they share many similarities. Given their shared similarities, perhaps the most surprising thing is that they don't talk to each other. They are like three side-by-side playgrounds, all with similar swing sets, slides, sandboxes, and climbers, but with arbitrary fences separating them.

The Gaussian filter (named after mathematician Karl Gauss) that is used for GMSK creates a rounded, normal distribution waveform, which results in a narrower and more efficient bandwidth signal. The Minimum Shift Keying shifts the frequency of the signal to distinguish between zeros and ones.

The following illustration, to which I added red lines and arrows for clarity, shows the original digital signal and the resulting modulated carrier wave. You can see that a digital one was converted to one sine wave, while a digital zero was converted to 1.5 sine waves.

Why is GMSK a good choice for amateur radio? In Intro to D-STAR, George Zafiropoulos, KJ6VU, states that it's relatively simple, which translates to lower cost, uses bandwidth efficiently, and the resulting signals have constant amplitude so they’re not affected by amplifier nonlinearities, which translates to less noise, as he shows in the following diagram.

Another couple of acronyms are for the multiplex method, or how the signals are combined: D-STAR and System Fusion use Frequency Division Multiple Access (FDMA), while DMR uses Time Division Multiple Access (TDMA).

Here's another diagram derived from Intro to D-STAR that shows the digital format of D-STAR's 4800 bits per second (bps) signal, to which I added an example of my radio ID data:

When the digital voice signal is being transmitted, the 72-bit audio frames and 24-bit data frames alternate until the audio transmission is complete, after which a final 48-bit data frame is transmitted, signaling the end of the transmission.

Note: At the beginning of his presentation, George Zafiropoulos, KJ6VU, thanks John Hays, K7VE, Debbie Fligor, N9DN, and Dan Smith, KK7DS for content he took from their presentations, but he doesn't specify which content each contributed. So I'll thank them all, too, in case these diagrams originally came from them.

More murky soup: Because of all of these acronyms, you'll see System Fusion sometimes referred to as C4FM or C4FM-FDMA. I told you the alphabet soup can get murky!

1d) Personal access points (hotspots)

This is where it gets really exciting and fun, at least for me. One more piece of the puzzle is figuring out how to get onto the digital system when you're not within radio range of a digital voice repeater.

Fortunately, there are innovative hams creating hotspots and software that enable a ham with internet connectivity to link directly to reflectors or DV repeaters, bypassing the need to transmit from the radio to a DV repeater first. Basically, these devices act as your own personal digital voice repeater and gateway.

There's another nice aspect of the hotspots: since you link directly to a reflector via a hotspot, you don't tie up a DV repeater the way you would if you use your radio to send a command to the repeater to link it via its gateway to a reflector for your personal call on that reflector. This may make a hotspot interesting even for someone who lives within range of a DV repeater.

Understanding repeater and reflector modules

When you use a DV repeater, or link to a reflector, you use a specific module (a.k.a, port, node, or room), for example, module C on reflector REF001 = REF001 C, which, by the way, can be a lively gathering place for international chats.

The agreed upon practice for naming these modules is to add a letter designating the individual port after the main callsign for the system. Regardless of the length of the main callsign, the port designation is always placed in the 8th character position, preceded by as many spaces as necessary to fall in the 8th position.

Exception: When you send a command to link to a repeater or reflector, the module letter is placed in the 7th position and the letter L is placed in the eighth. More on this later.

D-STAR repeaters typically have three modules for voice, and possibly an additional one for data. A repeater can be set up to use modules in various configurations, so you should double check the setup of any repeater you're going to be linking to; however, often they're used as follows:

Gateways are computer servers connected to repeaters that run software that enables them to act as "gateways" to the internet, enabling the repeaters to link to other remote repeaters and reflectors. They also enable hotspots to link to the repeaters via the internet.

Module G = Gateway

Here's a simplified view showing DV HT1 transmitting to and receiving from module C, while DV HT3 is using the gateway:

Note: When you link to a repeater via a hotspot device, the frequency of the module doesn't affect your ability to link, since you're linking via the internet, not RF. However, it still does matter since it affects who can hear you among the hams who are monitoring the repeater via RF. If you want to chat with hams who are monitoring the 70 cm module, you need to link to that module.

Trust system. Some U.S. gateways are registered with the USRoot Trust system (sometimes referred to simply as US Trust or the Trust system), which is how callsign routing and other information is synchronized between the repeaters within the trust system. That way, for example, the entire system knows which repeater you were most recently active on.

Reflector modules are like chat rooms

Reflectors are a type of network server, and you can think of their modules as chat rooms:

REF and older generations of XRF reflectors have five modules, A - E, where E is a special module for echo tests.

XLX reflectors, DCS reflectors, and the latest 4th generation of XRF reflectors each have up to 26 modules, A - Z.

&bigstar; The Wild West of amateur radio!

[ Starred notes are alerts about potential wrong turns! ]

Digital voice, especially in the area of hotspots, is like a Wild West frontier of amateur radio. There's lots of experimentation going on, which means both excitingly rapid progress as well as some abandoned dead-end branches of exploration.

One of the challenges of trying to learn all of this, especially for someone like me who isn't a SuperNERD, is that while there is some quite good information available online scattered around in various places, there's also some outdated and incomplete information out there. Because of this, I found it unfortunately much too easy to take some wrong turns along the way.

In this article, I share mostly the right turns I've taken, but will warn about a few potential wrong turns, like …

Be aware of the date!

Be aware of the date (or lack of date) of any material about digital voice radio that you come across, including this article (see the "last updated" date at the top). In an area that's changing this rapidly, information can be quite time sensitive.

If the material is much more than a year or two old, it may contain information that is wrong or at least partially outdated. Even if it's still technically correct, it may have been superseded by more current information posted elsewhere.

Example: There is a tip that is still live online today (as of Dec 2017) in an undated D-STAR article ranked highly in search results—which I mistakenly presumed meant it was relatively current—claiming that you can access X-Reflectors with the DVAP or DV Dongle devices running the DVAPTool or DVTool software by making changes to the "hosts" file [²].

I didn't even know what a hosts file is when I first came across that tip. After figuring out what it is and how to change it, I spent a lot of frustrating time trying to make the workaround actually work before learning via another website that an update made to the DVAPTool and DVTool software in early 2012 intentionally blocks that workaround [³]. That tip is more than five years out of date!

Fortunately, I was able to use what I learned about the hosts file for another task , so not all the time I spent on this wrong turn was wasted.